I’m thinking there is a bad bacteria in there so I cleaned out most of the existing sand and put in brand new sand. I did sprinkle a little of parents sand around on top (just a handful) but this appears to be a bacterial issue. Nothing on the body so something internal. I have been using...
I just went in and found ths. Lost one yesterday too. Not sure what’s going on at this late stage. He was fine at noon feeding. Going to do a large water change.
Your tank should be cycled by now. Do as Colin_T instructs. I don’t think you need to begin completely over again. Perhaps get some Tetra Safe Start + and add to the filter. Don’t use Prime for 24 hours after you do this. You can then start using Prime again to protect your fish after that...